Roof construction for commercial, residential, and industrial buildings, must meet performance standards generated by legislation primarily at the state and local levels through the adoption of state and local building codes which set forth very specific performance factors. In general, these performance standards are divided into two broad areas: (1) Sloped roofs, generally 30 degrees or greater from horizontal and (2) Flat roofs, 0 degrees to 30 degrees slope from horizontal.
The performance standards for flat roof construction vary slightly from area to area but generally conform to the following:
1. Vertical Load Strength: A roof must be able to carry a total load consisting of dead load plus live load and satisfy legislated design or performance values for the materials being utilized in the roof assembly. Example: Conventional steel roofs manufactured from 50,000 to 60,000 psi steel must not be stressed under working conditions beyond a flexural tensile stress of 20,000 psi.
2. Live Load Deflections: While supporting the designed dead load (weight of steel, built-up roof and insulation) the roof must not deflect under live load application more than 1/240th of the distance between the support members or less deflection as dictated by local building codes or other considerations.
Example: A roof supported by members 6′0″ on center must not deflect more than 6′0″×12 in./ft.× 1/240 equal 0.30″ under live load application. Live loads will vary in different climate areas from 20 pounds per sq. ft. to 60 lbs./sq. ft., depending upon weather conditions.
3. Wind Up-Lift Resistance: Typically, the roof must resist negative and positive pressures applied to it and remain structurally serviceable. Performance values for this standard vary depending upon geographical areas, but in general, range from 30 psf uplift resistance (equivalent of 100 mph winds) to 90 psf uplift resistance (equivalent of 188 mph winds).
Typical steel roof assemblies have utilized sections formed from mild steel in patterns normally referred to as “Type A”, “Type B”, “Type AB”, and the like. The common feature of the sections is a wide flat surface element, formed between stiffening ribs that provide the stiffness and strength to the section. The steel sections, supported by purlins, have been designed heretofore to meet strength requirements specified by building codes. The flat surfaces have been employed to provide a supporting surface for one or more layers of sheet material comprising a single board serving to insulate and provide a surface to which waterproof covering was attached.
A typical “Type A” section, for example, provides a flat portion of approximately 5 and ½ inches wide between 1 and ½ inch deep stiffening ribs spaced six inches apart. The “Type B, AB” and other sections are similar in profile to a Type A section except that the flat portions between stiffening ribs is progressively reduced in width to create a closer spacing of the stiffening ribs, increasing the load capacity for a given span. However, the width of rib openings on the top surface of the sheet, for example of a Type B section is greater than that of a Type A section.
The most efficient light gauge steel sections from a strength standpoint are those that have the greatest number of stiffening ribs per unit of width; the ultimate, being the symmetrical rib pattern sections, which have an equal distribution of steel above and below a neutral axis lying in a plane passing through the center of the sheet and disposed parallel with upper and lower surfaces of the sheet.
It may be mandatory in some cases, or desirable in other cases, for economy reasons, to utilize the roof assembly as a structural diaphragm to reinforce a building against lateral loads created by earthquake shocks (seismic), explosion forces or wind. In such application, the roof assembly is considered to be the plate web of a girder oriented in a horizontal plane with the perimeter members of the building serving as the compression and tension chords of the girder.
The diaphragm (plate web) strength of a given roof assembly is evaluated in terms of its ability to transfer diagonal tension stresses, which involves consideration of the shear resistance of the assembly, and in-plane deflection (referred to as “diaphragm deflection”), which is governed to a large extent by the “diaphragm stiffness” of the steel panel sections that are utilized. Diaphragm stiffness is related to the ability of the steel panel sections to resist distortion under load.
Since the flexural strength of a steel panel section is, to a large degree, a function of the depth of the section, it is naturally opposed to the reduction of depth (approaching a thin plane of steel) that contributes to diaphragm strength. The most efficient roof assemblies, from the standpoint of diaphragm strength, are those that can provide adequate flexural strength, utilizing steel sections with the maximum degree of effective steel in the diaphragm stress plane. Diaphragm stiffness increases proportionally to increases in the yield strength of the steel that is utilized, hence, steel sections made of high tensile steel are more effective than those made of mild steel.
Heavy gauge, mild steel (for example, 22 gauge, 20 gauge and 18 gauge with a stress limit of 20,000 lbs. per square inch) is generally employed in the manufacture of Type A and similar flat profile sections. This has been due to the fact that heavier gauges are necessary to satisfy the minimum steel thickness to element-width ratios that govern the design of light gauge steel sections. On the other hand, the symmetrical rib pattern sections have smaller unit-width elements and hence can utilize the more effective high tensile strength steel in lighter gauges providing greater working strength per pound of steel.
Asphalt built-up roof coverings usually consist of several layers of asphalt-saturated felt with a continuous layer of hot-mopped asphalt between the layers of felt. The top layer of such a roof covering may consist of a hot mopping of asphalt or coal tar pitch only, a top pouring of hot asphalt with slag or gravel embedded therein, or a mineral-surfaced cap sheet embedded in a hot mopping of asphalt.
Built-up roofs cannot generally be applied directly to steel roof sections and consequently an underlayment of substrate material has typically been installed after the steel roof sections have been secured in place. There have been embodiments which have employed a single sheet of underlayment material generally referred to as “rigid roof insulation board”. However, the insulating efficiency of the rigid board insulation is generally directly related to the density of the materials of which it is constructed, lighter density materials providing proportionally better insulation for a given thickness. Strength characteristics of these boards are inversely related to reductions in density. Accordingly, the lighter the density of these boards, the less the strength. Since “rigid insulation board” has heretofore been used over steel to provide a suitable base for roofing as well as insulation, the board had to be manufactured in densities that would compromise the minimum requirements for strength versus insulation values. Typical of compromised situations, the “rigid insulation boards” have been made to be adequate, but under the circumstances could not be fully efficient in the performance of either function, i.e., providing thermal insulation and strength.
U.S. Pat. No. 4,736,561 to Lehr et al., U.S. Pat. No. 4,783,942 to Nunley et al., U.S. Pat. No. 4,601,151 to Nunley et al. and U.S. Pat. No. 4,707,961 to Nunley et al. disclose horizontally disposed multi-layer flat building roofs.
Steel framing can be used to build purlins and trusses like conventional framing for both flat and sloped roofs. Roof trusses made of steel framing resemble wood framing with rafters and ceiling joists formed of C-shaped studs. A ridge member constructed of a C-shaped stud inside a track section connects the rafters. In a conventional wood or steel framed house having peaked roof sections, the rafters are perpendicular to and rest on the load-bearing walls. The end walls and interior walls parallel to the rafters are typically non-load bearing.
A number of approaches have been proposed to utilize steel roof trusses. In U.S. Pat. No. 2,541,784 issued to H. S. Shannon, “C” or “U” shaped sections are used for the bottom chord member as well as the top chord members of a building truss.
U.S. Pat. No. 4,435,940 issued to Jeanne A. Davenport, et al. and U.S. Pat. No. 4,982,545 to Gustaf M. Stromback describe truss arrangements wherein the horizontal, bottom chord section of a roof truss is formed from a U-shaped section of sheet steel. In the Stromback patent the ends of the legs of the U are tightly folded back to form a double thick edge. The top chords of both the Davenport and the Stromback patent are formed of inverted U-shaped sections having flanges projecting outwardly from the ends of each of the legs to provide greater rigidity.
U.S. Pat. No. 5,463,873 to Early, et al. discloses a metal roof truss wherein, the bottom horizontal chord piece and the top chord pieces are of substantially uniform shape and cross-section. Both the bottom and top chord members include a radiused or rolled hem at the end of the legs. Further, one or more stiffening ribs are formed in the side walls of the chord members.
The trusses are typically arranged parallel to each other with 8″, 16″ or 24″ between their respective centers, depending upon the load characteristics that the roof must accommodate. A sheathing material such as plywood or OSB is then fastened to the upper chords of the trusses using nails, screws or other mechanical fasteners to form the roof surface. To prevent the trusses from twisting or moving laterally, small pieces of wood or metal, known as purlins, are commonly nailed between adjacent trusses. Insulation is sometimes installed between the trusses and sheathing, drywall, plasterboard, etc. may then be applied to the bottom of the joists to form a ceiling for the space located under the roof truss system.
The outside dimensions of the metal framing members and studs, and the weight or gauge of the member or stud, vary. Typically the members are fabricated to be approximately 4 inches wide by 2 inches deep, corresponding thereby to the width and depth of wood framing and stud members, in which case the lips may extend ¼ to ½ inch from the sides of the studs. Eighteen to 20 gauge metal may be used for light gauge, residential construction and commercial wall construction. A heavier range of metal gauge is used in some residential and commercial framing and particularly in multiple story commercial construction.
Metal roofing framing members have been modified to include saw or punch slots, tabs and brackets intended to facilitate the interconnection of these studs and framing member to adjoining studs and framing members and/or to cross-bars and other non-framing members that serve to reinforce the studs and framing members. Known connectors, including bracket, plate and tie connectors, presently used to tie together and interconnect metal studs, are generally drilled and screwed on site. Drilling and screwing unsecured connectors pose a safety risk to the worker since the connectors tend to be small and light, and thus easily grabbed and spun by a hand drill.
It is known to place plywood or OSB sheathing on cold formed, light gauge steel roofing trusses. However, plywood and OSB are combustible.
U.S. Pat. No. 6,620,487 to Tonyan et al., incorporated herein by reference in its entirety, discloses a reinforced, lightweight, dimensionally stable structural cement panel (SCP) capable of resisting shear loads when fastened to framing equal to or exceeding shear loads provided by plywood or oriented strand board panels. The panels employ a core of a continuous phase resulting from the curing of an aqueous mixture of calcium sulfate alpha hemihydrate, hydraulic cement, an active pozzolan and lime, the continuous phase being reinforced with alkali-resistant glass fibers and containing ceramic microspheres, or a blend of ceramic and polymer microspheres, or being formed from an aqueous mixture having a weight ratio of water-to-reactive powder of 0.6/1 to 0.7/1 or a combination thereof. At least one outer surface of the panels may include a cured continuous phase reinforced with glass fibers and containing sufficient polymer spheres to improve nailability or made with a water-to-reactive powders ratio to provide an effect similar to polymer spheres, or a combination thereof.
U.S. Pat. No. 6,241,815 to Bonen, incorporated herein by reference in its entirety, also discloses formulations useful for SCP panels.
U.S. patent application Ser. No. 10/666,294, incorporated herein by reference, discloses a multi-layer process for producing structural cementitious panels (SCP's or SCP panels), and SCP's produced by such a process. After one of an initial deposition of loosely distributed, chopped fibers or a layer of slurry upon a moving web, fibers are deposited upon the slurry layer. An embedment device mixes the recently deposited fibers into the slurry, after which additional layers of slurry, then chopped fibers are added, followed by more embedment. The process is repeated for each layer of the board, as desired.
There is a need for an economical, easy to assemble, durable and non-combustible total framing and roofing system.